Still at odds with conventional galaxy evolution: the star formation history of ultradiffuse galaxy Dragonfly 44

Abstract

We study the star formation history (SFH) of the ultra-diffuse galaxy (UDG) Dragonfly 44 (DF44) based on the simultaneous fit to near-ultraviolet to near-infrared photometry and high signal-to-noise optical spectroscopy. In fitting the observations we adopt an advanced physical model with a flexible SFH, and we discuss the results in the context of the degeneracies between stellar population parameters. Through reconstructing the mass-assembly history with a prior for extended star formation (akin to methods in the literature) we find that DF44 formed 90 per cent of its stellar mass by z ∼ 0.9 (∼7.2 Gyr ago). In comparison, using a prior that prefers concentrated star formation (as informed by previous studies of DF44’s stellar populations) suggests that DF44 formed as early as z ∼ 8 (∼12.9 Gyr ago). Regardless of whether DF44 is old or very old, the SFHs imply early star formation and rapid quenching. This result, together with DF44’s large size and evidence that it is on its first infall into the Coma cluster, challenges UDG formation scenarios from simulations that treat all UDGs as contiguous with the canonical dwarf population. While our results cannot confirm any particular formation scenario, we can conclude from this that DF44 experienced a rare quenching event.

PEARLS: Low Stellar Density Galaxies in the El Gordo Cluster Observed with JWST

“…limits, we assume a R c -band mass-to-light ratio of 1.76 (corresponding to an r CFHT -band mass-to-light ratio of 2.1), taken as the median stellar mass-to-light ratio of spectroscopically characterized UDGs from Gu et al (2018), Ruiz-Lara et al (2018) andFerré-Mateu et al (2018) with Subaru photometry from Yagi et al (2016) (all corrected to Coma's luminosity distance of 102 Mpc, and consistent with dwarf galaxies with old, metal-poor stellar populations). This is largely consistent with results from spectral energy distribution (SED) fits of local UDGs (e.g., Pandya et al 2018;Buzzo et al 2022;Webb et al 2022), although the possible impact of attenuation by dust on their stellar mass measurements means that we do not use them in our averaging process. We conduct SED fitting (Sec.…”

“…Altogether, this evidence suggests that LDGs in El Gordo fell into the cluster relatively recently -they are largely similar to large isolated galaxies that have recently been accreted by the cluster. While many cluster UDGs at z = 0 are old (Gu et al 2018;Ferré-Mateu et al 2018;Ruiz-Lara et al 2018;Pandya et al 2018;Buzzo et al 2022;Webb et al 2022) some are young (Rong et al 2020;Buzzo et al 2022). On the other hand, our anlysis seems to suggests that most LDGs in El Gordo are young.…”

PEARLS: Low Stellar Density Galaxies in the El Gordo Cluster Observed with JWST

“…15 Specifically, we define LDGs as systems with stellar 2 ) limits, we assume an R c -band mass-to-light ratio of 1.76 (corresponding to an r CFHT -band mass-to-light ratio of 2.1), taken as the median stellar mass-to-light ratio of spectroscopically characterized UDGs from Gu et al (2018), Ruiz-Lara et al (2018), and Ferré-Mateu et al (2018 with Subaru photometry from Yagi et al (2016; all corrected to the Coma luminosity distance of 102 Mpc, and consistent with dwarf galaxies with old metalpoor stellar populations). This is largely consistent with results from spectral energy distribution (SED) fits of local UDGs (e.g., Pandya et al 2018;Buzzo et al 2022;Webb et al 2022), although the possible impact of attenuation by dust on their stellar mass measurements means that we do not use them in our averaging process. We conduct an SED fitting (Section 3.2) to determine the stellar masses of our objects, which are used to calculate the stellar surface densities for our selection.…”

“…Altogether, this evidence suggests that LDGs in El Gordo fell into the cluster relatively recently-they are largely similar to large isolated galaxies that have recently been accreted by the cluster. While many cluster UDGs at z = 0 are old (Ferré-Mateu et al 2018;Gu et al 2018;Pandya et al 2018;Ruiz-Lara et al 2018;Buzzo et al 2022;Webb et al 2022), some are young (Rong et al 2020;Buzzo et al 2022). On the other hand, our analysis seems to suggest that most LDGs in El Gordo are young.…”

The Supersonic Project: The Eccentricity and Rotational Support of SIGOs and DM GHOSts

“…Uncertainties and tensions remain, however, especially on the scales of faint dwarf galaxies (e.g., Bullock & Boylan-Kolchin 2017;Simon 2019;Perivolaropoulos & Skara 2022). From uncertainties such as the core-cusp challenge (e.g., Flores & Primack 1994;Moore 1994) to serious tensions such as the observed diversity of rotation curves compared to simulations (e.g., Oman et al 2015Oman et al , 2019, challenges to ΛCDM at low masses include not only tensions with observations (e.g., Webb et al 2022) but also discrepancies between different state-ofthe-art cosmological simulations (see Sales et al 2022 for a review). The ultra-faint dwarf regime is thus expected to be one of the most sensitive probes of models and simulations of structure formation that succeed at the scales of Milky Waylike galaxies and larger mass dwarf galaxies.…”